Make test duration more stable

Using `2.hours.ago - 1.hour.ago` gives a value that is slightly off from
3600 seconds (and also is a negative number - not a duration). `1.hour`
is more stable and simple to understand

spec/lib/gitlab/ci/pipeline/chain/command_spec.rb

@@ -345,7 +345,7 @@ RSpec.describe Gitlab::Ci::Pipeline::Chain::Command do
   describe '#observe_step_duration' do
     it 'adds the duration to the step duration histogram' do
       histogram = double(:histogram)
-      duration = 2.hours.ago - 1.hour.ago
+      duration = 1.hour
 
       expect(histogram).to receive(:observe).with({}, duration.seconds)
       expect(::Gitlab::Ci::Pipeline::Metrics).to receive(:pipeline_creation_step_duration_histogram)